R1:58884b94dc0b

Simplify construction and execution of Differential queries for "responsible" users

Summary:
Currently, when querying for responsible users (revisions where a given user is either the author or a reviewer), we do this:

- If the query passes a bunch of hard-coded special cases, use a special hard-coded UNION.
- Otherwise, use a very complicated JOIN/WHERE clause.

This is bad for several reasons:

- Tons and tons of hard-coding and special casing.
- The JOIN/WHERE clause performs very poorly for…